Field of the Invention
The present invention relates to a stationary induction apparatus, and particularly to a stationary induction apparatus including an electrostatic shield.
Description of the Background Art
When an impulse voltage such as a lightning surge intrudes into a stationary induction apparatus such as a transformer or a reactor, the potential distribution within a winding becomes steep as compared with the potential distribution proportional to the turn number, and then, it oscillates around the potential distribution proportional to the turn number. This phenomenon is referred to as potential oscillation. When the amplitude of the potential oscillation is relatively large, a large potential difference occurs between the electric wires located adjacent to each other within the winding, and between the windings located adjacent to each other, which may cause a dielectric breakdown. When an electrostatic shield is arranged adjacent to the winding, a capacitance between the windings becomes larger than the capacitance between the winding and the ground, so that the amplitude of the potential oscillation is reduced.
As a prior art document, Japanese Utility Model Laying-Open No. 60-113614 discloses a transformer including an electrostatic shield. In the transformer disclosed in PTD 1, an electrostatic shield is provided at both ends of the winding in the central axis direction. Each of the ends of the electrostatic shield on the outer circumferential side and the inner circumferential side is formed of a curved surface.
The electrostatic shield of the transformer disclosed in Japanese Utility Model Laying-Open No. 60-113614 includes, on the side opposite to the side adjacent to a coil, an electric-field concentrating area at each of its ends on the outer and inner circumferential sides. When the electrostatic shield is configured to have a relatively large radius of curvature at each of its ends on the outer and inner circumferential sides in order to suppress the electric field concentration at each of the ends of the electrostatic shield on the outer and inner circumferential sides, the electrostatic shield is increased in thickness, so that the stationary induction apparatus is increased in size.